Résumés

AMS (Anisotropy of Magnetic Susceptibility) is a geophysical method, which uses the electromagnetic properties of samples as a proxy of their fabric. Commonly used on hard rocks and cohesive sediments, the method has never been developed for unconsolidated sediments before. In this contribution, we present the usability of the AMS method on unconsolidated rocks and the new insights that AMS brings for the study of tsunami deposits, and for geomorphology. We have been working on the deposits of the coast of North Sumatra (Indonesia) because a devastating tsunami has struck these coasts and has redesigned the coastal environment and sediments distribution the 26 December 2004. Hence, this geographical setting has provided us with the necessary material for the study. We have carried out the analysis from 6 samples, which evidenced: (i) a first layer deposited by an uprush oriented to the SSW; (ii) a decanting phase; (iii) a layer deposited by a backwash oriented to the North; (iv) two other uprushes that deposited two units oriented to the SSW and the SSE. This research has proven that the AMS could be used on unconsolidated deposits, and that the orientation and an approximation of the energy during deposition could be inferred as well. It leads to new developments in marine and or fluvial geomorphology, even for unconsolidated sediments.

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1According to D.A. Clark et al. (1992), AMS is a fundamental data for geophysics and earth sciences. AMS stands for Anisotropy of Magnetic Susceptibility. It is a fast, non-destructive and low cost method for investigating rock fabrics. The AMS method describes the measure of the orientation of the magnetic ellipsoid generated during the exposure of a sample to a series of magnetic field (H). Traditionally in experiments, an AC electrical current induces H. For each measure, the induced magnetic moment (M) is proportional to H, with a relation described by a scalar k (the volume magnetic susceptibility) such as M = k.H. The volume magnetic susceptibility k depends on the enhancement of magnetic minerals that are present in a sample, and on the orientation of the sample. The anisotropy can be visualized in 3D thanks to a triaxial AMS ellipsoid. Three axis characterize the ellipsoid: Kmax, Kint and Kmin, with the following relation Kmax< Kint<Kmin, that are respectively the susceptibility maximum, intermediate and minimum axis (fig. 1). The volumetric fabric of the sample controls the latter axis, and the AMS is a tool for visualizing the orientation of these different axes. According to N. Hamilton and A.I. Rees (1970), the values of magnetic parameters are related to the primary sedimentary fabrics of natural sediments providing that (i) the angle between the plane of maximum-intermediate susceptibilities with the horizontal plane is less than 20°, and (ii) the shape factor q ranges between 0.06 and 0.7. Although the AMS technique relies on the use of an external magnetic field for its measurement, the methodology and concepts behind this technique are more akin to a traditional petrofabric approach than they are to paleomagnetic works.

2Up to present, AMS has been a common technique for consolidated and hard rocks (Argenton et al., 1975; Ellwood, 1975) and more especially volcanic rocks (e.g., Palmer and McDonald, 1999; Canon-Tapia and Pinkerton, 2000). AMS measurements have started in the early 1960s with studies of granitic rocks (e.g., Balsey and Buddington, 1960). The AMS method has been essentially used to determine the magma flow dynamics and the emplacement mechanisms, through the establishment of a relation between AMS patterns and tectonic structures. The AMS method has also numerous applications for marine and lacustrine deposits (e.g., Granar, 1958; Park et al., 2000) and for aeolian deposits (e.g., Bradak, 2009), and for tectonic applications (e.g., Borradaile and Henry 1997; Zak et al., 2009). Eventually, the method has been extended to rocks heated in laboratory, for which the heating process can enhance in some case the original AMS signal (Henry et al., 2003). Despite these technical evolutions, the studied material was mostly hard-rock or cohesive sediments. We based the present research on the hypothesis (Rees, 1965) that the AMS method could be extended to unconsolidated sediments, proving that we have an appropriate sampling technique. This contribution aims at presenting the results of the AMS method applied to unconsolidated sediments: tsunami deposits, and it highlights the opportunities that it brings to geomorphology.

3Scientists conventionally display AMS data on a circular plot, which represents the projection plan of the lower hemisphere of AMS tensor (Kmax, Kint, Kmin). Therefore, these tensors are represented as a single point when the tensors meet the boundary of the lower hemisphere. They correspond to a cyclographic trace when the plan defined by Kmax and Kint meet the lower hemisphere. The angle of this plan is modified each time a measure is done (fig. 2, see 1, 2 and 3). The synthetic diagram of these series of measures (fig. 2, lower part) provides us with the tilting of the Kmax and Kmin tensors, Kint providing complementary information on Kmax. The position of Kmax provides the declination in degree of the sample, i.e. the orientation of the sample. The vertical inclination of the grain is inversely proportional to the distance between the position of Kmax and the centre of the circular diagram. The same concepts apply to Kint. Finally, Kint gives complementary information on Kmax and Kmin, as a confirmation of the data collected by Kmax and Kint. Kmin also provides information on the grain orientation when the tensors Kmax or Kint are vertical, and then do not provide information on their orientations, since their cyclographic trace appears at the centre of the diagram.

4The preamble of this research is based on the existence of a preferred orientation for most of grains in a sediment body. This orientation is due to the forces that have acted during sediments deposition. Therefore, the AMS, governed by the volume’s distribution should help retrieving this information. G.B. Jeffreys (1922) has conducted the first theoretical and experimental researches on grain fabrics and on their relations to the sedimentation mechanisms, to the flow power and its direction. For well-sorted sands (which is the case for our example in Sumatra), the planar lamination of the deposits comes from the superposition of layers of similar grain size. AMS studies benefit the analysis of rock fabrics, grain orientations in rocks, magmatic flow paths orientations, etc. (e.g., Rochette et al., 1992), because grain alignments result in anisotropy of magnetic susceptibility. Therefore it is a good proxy of sediment fabric, which allows the reconstruction of flow directions during settling phase. Studies of the magnetic fabric or experimentally deposited sediments indicate that the magnetic grains are mainly oriented parallel or close to the bedding plane, with their longer axes parallel to the flow direction, and with an imbrication, when existing, dipping upstream (Hamilton and Rees, 1970; Taira 1989). The reader will note that this relation is only true for sand grains.

5Tsunamis are events that can have catastrophic imprints on human settlements and the environment as well. The recognition and the analysis of tsunami deposits is of wide interest, because it provides an insight on paleotsunamis and potential hazards (Goff et al., 2001; Felton and Crook, 2003; Scheffers, 2004; Paris et al., 2007). Tsunami can displace large amount of sediments, impacting hundreds to thousands of kilometers of shorelines. Although the phenomena can last for a couple of minutes only, the environmental and sedimentary changes can span in geological times (e.g., Scheffers, 2004; Bondevik et al., 2005; Williams et al., 2005; Goff et al., 2006). Tsunami deposits have also been recently named tsunamiites (Dawson and Shi, 2000; Dawson and Stewart, 2007; Shiki et al., 2008). The term tsunamiites was most probably created in 1988 by the Chinese Gong Yiming. It was firstly used in a publication by T. Yamazaki et al. (1989) to describe a Miocene tsunami deposit of boulders, and it has become widely accepted with the last book on tsunami deposits named ‘tsunamiites, features and implications’ for instance (Shiki et al., 2008). Tsunamiites can comprise material mainly extending from large boulders (e.g., Kelletat et al., 2004; Scheffers, 2004; Goff et al., 2006; Paris et al., 2009) to sand deposits (Paris et al., 2007).

6D. Sugawara et al. (2008) have proposed a distinction in tsunamiites, between deposits that have sedimented in subaerial and other that have settled in subaqueous environments, the latter being divided into sea floor and enclosed environment deposits. Submarine deposits seem difficult to identify. The erosion of fine sediments at the surface of the sea-bottom, changes in diatom assemblages (Okamura et al., 2004), and the intercalations of coarser sediment particles within muddy layers (Fujiwara et al., 2000; Takashimizu and Masuda, 2000) are submarine sediments proxies of tsunamis. Underwater, the suspension of particles dominates the transport process; therefore free falling of sedimentary particles from suspension may create an upward-fining sequence in submarine tsunamiites (e.g., Tada et al., 2002). However, submarine evidences of tsunami deposits remain scarce, because of data accessibility (Paris et al., 2010b). Observations of tsunamiites in basin and/or in lacustrine environments have identified distinct sand layers in silty to clayey sediments. The units below the sand deposits are typically partially eroded, because of the traction flow effects induced by the tsunami (Bondevik et al., 1997). Such deposits have been observed on the Scilly Isles, seemingly because of the 1755 Lisbon earthquake and associated tsunami (Foster et al., 1993), in Japan (e.g., Hirose et al., 2002; Nanayama et al., 2002).

7On dry land, tsunamiites can be divided between uprush features and backwash units, although the distinction is not always simple. The most characteristic features of uprush units are sand-sheets that cover the sub-horizontal areas (Atwater, 1987; Dawson et al., 1988; Long et al., 1989, 1990; Minoura et al., 1996, 2000; Paris et al., 2007; Wassmer et al., 2007; Paris et al., 2009). Vertical grain-size variations within the sand layer can also form graded bedding, with often coarse sand at the base and fine sand at the top (Benson et al., 1997; Dawson et al., 1988, 1991; Gelfenbaum and Jaffe, 2003; Wassmer et al., 2007). The layers tend to diminish in depth with the distance to the sea, forming wedges (e.g., Goff et al., 2004). High-energy tsunamis, such as the one that struck the Indian Ocean in 2004, can also erode coral reefs and depose coral fragments inland. Although these deposits are mostly limited to the first hundred meters to the shoreline, they can be numerous and their thickness can reach several meters (Scheffers, 2004; Paris et al., 2009). Backwash units have been recognized in inhabited areas from the presence of human artefacts in the deposits (Barthomeuf et al., 2010). The backwash often erodes the substratum and the newly deposited tsunamiites before depositing a poorly sorted sandy mud (Sato et al., 1995) or fine grain sands. Bent plants (Nishimura and Miyagi, 1995), cobbles (Nanayama et al., 2000) indicate the direction of the backwash of a tsunami, which is not always the opposite direction of the run-up.

8However, in the absence of such proxies - for paleotsunamis, or tsunami with a low backwash energy for instance -, the AMS method on unconsolidated sediments can inform us on the flow direction and the flow energy. Scientists have carried out identification of tsunami and paleotsunami deposits all around the world, in Europe (e.g., Dawson et al., 1988), in Northern America (e.g., Clague and Bobrowsky, 1994), in Southern America (e.g., Cisternas et al., 2005), in Central Pacific (e.g., Moore et al., 1994), in Oceania (e.g., Goff et al., 2001; Dominey-Howes, 2007), in East Asia (e.g., Minoura and Nakaya, 1991), and in South-east Asia (Wassmer et al., 2007, 2010), using geomorphological, sedimentological, stratigraphical, archaeological, palynological and paleontological evidences. Despite these numerous researches, which have peaked after the December 2004 tsunami, methods and tools haven’t evolved much. C. Gomez et al. (2010) have applied geophysical investigation – GPR (Ground Penetrating Radar) – on sand deposits, but this method presents important technical limitations, and it doesn’t help with the distinction between a tsunami deposits and another sand-sheet, or between uprush and back-wash units. C. Chagué-Goff (2010) has re-explored the potentiality of chemical signatures for distinguishing tsunami deposits, but the decay of chemical signatures vary with time and environments, so that the method is hardly applicable. B. Mamo et al. (2009) and R. Paris et al. (2010) have respectively re-explored foraminifera assemblages and nanoliths, and they have used them as proxies to identify seawater flooding. These methods are also constrained by the presence of a proxy and it is not possible to reconstruct with precision the different stages of the shuttle movement from the sediment analysis. In this context, the AMS method applied to unconsolidated deposits come anew in this research field (Wassmer et al., 2010), and it offers the possibility to determine uprush from back-wash sandy tsunamiites. The present contribution aims at correcting and improving the 2010 vanguard paper, by bringing a more precise insight on the method, by voiding the statistical incorrectness, which brought mischievous results in the previous paper (because of the averaging process).

9The Indian Ocean tsunami of 26 December 2004 devastated about 180 km km of the Aceh coast (Northwest Sumatra), mainly between Banda Aceh and Meulaboh (fig. 3). The tsunami eroded the coast back (e.g., Wassmer et al., 2007; Paris et al., 2009) up to 500 m and it removed a large number of beach landforms and sand dunes. The tsunami was generated by the largest earthquakes ever recorded (M9.3 on Richter open-scale), which was produced by the largest known earthquake rupture (Lay et al., 2005). The main-shock rupture started 2 mn before 8:00 AM, displacing up to 30 km3 of seawater (Bilham, 2005). This tsunami has materialized on shores by 15-35 m m high waves (Lavigne et al., 2009), with a decrease towards the South. The sampling area is located on the sub-horizontal coastal plain of the Kajhu Perumnas areas, east of Banda Aceh. The topography is very flat and limited by an east-west trending linear shoreline(fig. 3).The whole zone of the sea front was constituted by a draught board of shallow fish or shrimp breeding basins. These artificial depressions with an average depth of 1 m m had a clayey bottom and were bordered by sandy dykes. Beyond 1,5 km km landwards, very shallow coastal lagoons were present in some places. During the December 26, 2004 event, the first tsunami wave, which flooded the littoral zone, eroded the sea front dunes. The following waves easily crossed and leveled the dune field before spreading on the coastal plain, which was then under the direct influence of the surging water (Wassmer et al., 2007, 2010). The second wave has been the most important and it has reached 13 m-high in the study area (Lavigne et al., 2009). In the study area, all these depressions (breeding basins, shallow lagoons) were flooded by the arrival of the successive tsunami waves without significant backwash, except after the first wave, before the end of the flooding, according to eyewitnesses (Lavigne et al., 2009). This particular behavior, probably due to the flatness of the coastal plain, differs drastically from the normal swash cycle of each wave (uprush/flooding-slack/backwash) and must be taken in account to interpret the sedimentary signature.

10We have collected the samples with 20 mm wide cubic plastic non-magnetic boxes along vertical sections of the pits dug out in the tsunami deposits. The boxes were carefully pushed manually into the moist sediment along a horizontal direction, normal to the vertical sections of the deposits. In order to avoid air compression in the box during sampling, which could have induced modifications in the grains-structure, a small hole was previously drilled at the bottom of each box to allow air escape. Before sample removal from the sediment, each box was oriented (fig. 4). The sampling boxes were sealed after removing from the sections to avoid desiccation and, by then, disturbance of the sedimentary fabric. Samples were collected in each layer of the deposits according to visual facies changes. Thin layers were not collected because of sharp changes in sediment textures within the size of the sampling boxes. Sampling of some sections was not continuous or not complete (like top sequences of Section F for instance). For this contribution, in a first time, we have analyzed 6 samples from a pit located in a former breeding basin along the pre-tsunami coastline in Kahju area (fig. 3). We collected the 6 samples at 35, 19, 14, 10, 6 and 4.5 cm cm from the roof of the deposit. In a second time, at the laboratory of Lille University (CNRS-UMR 8157 ‘Geosystèmes’), we have processed the samples with a Kappabridges® KLY-2 (fig. 5). It consists of a pick-up unit control connected with the DPU1 data processing unit and a computer to save the data. We have analyzed each sample in 15 different directions in order to reconstruct the electromagnetic ellipsoid and to determine the magnitude and direction of the different tensors (Kmax, Kint, Kmin).

11The AMS analysis of the 6 samples has provided us with the vertical tilting data of the Kmax, ranging from 8° to 20°, with an average of 10° and a standard deviation of 8.3°, which proves the heterogeneity of the dataset. Indeed, 4 data displays a Kmax tilting angle comprised between 14° and 20°, whereas two data describe sub-horizontal patterns with angles of 0° and 1° (fig. 6). These angles are the direct expression of the deposition angles of sand grains, therefore they are essential to understand the conditions of deposition. The data has shown two preferential orientations, with 2 samples oriented towards the north (samples 3 and 4), and 4 samples oriented between SSE and SSW. The AMS analysis provided some additional parameters that are useful for better understanding the characteristics of sediment emplacement processes. L is the lineation value and it reflects the importance of the traction during emplacement. F, the foliation value reflects the importance of the decantation processes. Fs is the alignment parameter. Linked to the shearing exerted by the flowing water on the bottom of the water column, it parallels the evolution of L and F (tab.tab. 1).

12Chronologically, we can infer from the obtained dataset, that the first and the lowest unit in Sections B, constituted of reworked sand from the littoral dunes field could represent the sedimentary record of the first tsunami wave surge (Wassmer et al., 2010). This material corresponding to 42% of the sediment volume in this area has been deposited along a SW orientation, with a strong tilting of 20° (sample 1). The second sedimentary unit constituted by a fining upwards sequence corresponds to the second wave that reached 15 m m in height in Kajhu (22.5% of the sedimentary signature). At the base, the material is coarse and contains numerous bioclasts and small rip-up clayey clasts. Due to this coarse material, sampling was not possible in this layer. The deposit evolves upwards to fine sands (samples 2 and 3). Base and top of this unit seem to have been deposited in two opposite directions but the tilting was only 1° and 0° (samples 2 and 3), so that the grain deposition was horizontal. This has been confirmed by the Kmin, which is located at the center of the cyclogram (fig. 6), so that the Kmin tensor is sub-vertical. Unit 3 corresponds to 4.5 cm cm of coarse sand layer displaying a 14° northward tilting (sample 4). The sedimentary contribution of this unit reaches 11.25% of the total amount of the material brought by the tsunami. Unit 4 (sample 5) displays a 17° tilting to the SW had a contribution of 5%. Finally, unit 5, the closest to the roof of the deposit (sample 6) displays an 8° tilting, oriented to the South.

13According to the position of the sample pit to the sea (fig. 3), the cyclogram has described:

14(i) A first uprush (sample 1) oriented to the SW that emplaced a thick unit of coarse sand (tab.tab. 2). The weak foliation factor (F) attests for a traction-dominated settling.

15(ii) A second uprush (samples 2 and 3). It corresponds to a fining upwards sequence. The base was not sampled but the coarse material and the presence of numerous rip-up clasts pleads for a strong energy at first. Samples have been collected in the fine material emplaced by the “tail” of the wave (tab.tab. 2). The very low energy did not allow a clear tilting to occur. Base (sample 2) and top (sample 3) are respectively deposited in a SSW and a SE orientation. This variation is probably due to the interference between a direct and a reflected wave. Lineation factor is weak while foliation increases correspond to a deposition process dominated by decantation (for the upper part of the deposit).

16(iii) A backwash (sample 4) oriented to the NNE that emplaced a 4.5 cm cm thick layer of sand. The tilting (14°) and the increasing of the lineation factor reflect traction-dominated emplacement.

17(iv) Two uprushes (samples 5 and 6) oriented to the SW and S respectively. This variation of the incidence angle is probably due to the arrival of a reflected/refracted wave. The mean grain size is the same for the two waves and the smallest of the whole deposit but tilting is one of the stronger of the whole section with 17° for sample 2 which is characterized by the maximum value of the alignment factor that confirms strong bottom currents during emplacement. The second uprush that corresponds to sample 1 shows a tilting of 8°. The sedimentation process is dominated by settling as attested by a high value of the foliation factor.

18This interpretation is inferred from the waves orientations, the mean grain size, but also from the declination angles of the grain deposited. The highest is the energy or the velocity of the waves that brought the sand, the higher the tilting is. Therefore, uprush waves usually present the highest declination angles, although the dataset has indicated backwash oriented units with a 14° tilting.

19Application of AMS method to the 2004 tsunami sandy signature has been of interest since we have a thorough knowledge of the whole tsunami wave train on this area of Kajhu from field and eyewitness evidence. The method that we have presented in this contribution has produced results that have been compared with eye witnesses reports of tsunami waves orientations, with, tilting of building structures and trees, and with orientations of scorch marks (Wassmer et al., 2010), as described by F. Lavigne et al. (2009). Data provided by AMS confirms the results that we have previously described on wave dynamics and it also provides further details on the processes involved in sediment emplacement. It allows the analysis of original parameters that could not be investigated by traditional methods. Thanks to the AMS method, we have worked on each individual deposit interval identified in the field from sedimentary evidences, i.e. fining upwards sequences with sharp limits on base and top that corresponds to the sediment mass brought by a single wave. We have evidenced the variation of the orientation of the water flow during emplacement of each individual sequence and to differentiate the sediments deposited by uprush from those deposited by backwash. The evidence of a tilting to the North, related to a backwash process, for sample 4 is surprising here because eyewitness testimonies have reported that no backwash has occurred in this area (except after the first wave) and that the water pushed landwards during the entire event (Lavigne et al., 2009). This data attests that the backflow begun before the last waves reached this zone located near the former coastline. Eyewitnesses were provided by people that took refuge in the remaining houses far from the seashore where all the buildings were wiped out by the tsunami. This new insight implies a spatial restriction of the information given by eyewitnesses to the distal zone only, i.e. far from the previous seashore.

20This work has proven that the AMS method is a reliable proxy of the deposition conditions of sandy deposits. However, this method has intrinsic difficulties, which we need to address by protocol explorations. Five weak points are exposed:

21- Even if this method is not difficult to implement, it requires extreme care during the sampling process. As it is impossible to sample in non-cohesive dry fine material, sampling protocol requires moist sediments. Their cohesion prevents fabric disturbance during sampling process. The very thin boundary layer disturbed by the sampling box (≤1 mm) make this AMS well adapted to fine sediments.

22- The size of sampling box constitutes one of the main limits to this method. When the layer deposited by a sedimentary process is thinner than the size of the box, i.e. 20 mm., sampling is impossible. In Banda Aceh, due to this limit, it was impossible to sample the uppermost sequences of some sections in our study area because their sicknesses were below the box size and therefore we were not able to analyze these layers with the described technique. Future investigation techniques may allow overcoming this limit.

23- Coarse material, i.e. bioclasts or rip-up clasts contained in fine sediments, can disturb the sand fabric during the sampling when the box is pushed in the sediment. Due to this limit, on the investigated pit, the base of the second fining upwards sedimentary interval corresponding to the second uprush sequence identified cannot be sampled.

24- Another limitation of the method concerns poorly sorted sediments with an important fraction of coarse volcaniclastic particles. The individual magnetic susceptibility of the coarse volcanic grains can bias the bulk volume susceptibility of the whole sample itself.

25- The measured tilting (Kmax) of a sample reflects the sediment fabric and allows us to retrieve the flow direction during deposition. The absence or weakness of tilting is hard to interpret in term of flow direction. A weak tilting measure is related to a decantation dominating process during emplacement but might be a consequence of the difficulty to maintain the box perfectly horizontal during the sampling and therefore might induce a bias in the data interpretation. For instance, sample 2 corresponding to the lower part of the second wave tail deposit, i.e. second fining upwards sequence; the measured tilting is 1°. This weak value can be attributed to a very slow bottom current but at the same time could result from the box horizontality incertitude during sampling. We faced this problem during our study in Banda Aceh where we partially explained the reduction of the grain tilting by potential back and forth water movements (seiche effect) inside the breeding ponds (Wassmer et al., 2010).

26Hence, the method opens up new opportunities for understanding hydrodynamic conditions and for reconstitutions of flow directions during deposition processes (uprush or backwash). Although, we developed this contribution from unconsolidated deposits, AMS can bring new insights to old and indurated tsunami deposits using a driller for the sampling.

27The efficiency of the AMS method has not to be proven. It has been used for a long time by geologists on hard rock, i.e. granitic rocks to reconstruct orientation of the progression of magmatic rocks or volcanic rocks to retrieve lava flow direction. On lacustrine or marine sediments, AMS was applied to research the depositional conditions (traction or decantation). Applied to unconsolidated fine material, this method can be helpful for researchers to investigate various fields of geomorphology. In fluvial geomorphology, when the topography is not a good indicator (Mississippi for instance), AMS can help retrieving flow orientations from former channels or terraces over floodplains. It could come as a complementary approach to the study of dunes and bars structures (e.g., Reesink and Bridge, 2007, 2009), in order to link these deposits with flow velocity and hydrological power. Openings in volcanic geomorphology are also promising. AMS can bring new insights on deposition processes of block-and-ash flows for instance, which are still difficult to assess, despite improvements in subsurface geophysical investigations (Gomez et al., 2008, 2009). AMS could also provide information on the differentiations of complex layering in lahar deposits, that have been proven more complex than expected from outcrops analysis (Gomez and Lavigne, 2010). The AMS method has proven its compatibility with unconsolidated sediments, as long as the sediment is wet, and as long as the sampling protocol is strictly followed. This method brings a new way of working on tsunamiites, with the possibility to retrieve the flow orientation and energy, not only for the whole deposit but for each single wave constituting the tsunami wave train, as long as corresponding deposits are >2 cm cm thick. Within each sedimentary interval that corresponds to the sediment contribution of a single wave, it is possible to evidence flow orientation variations from base to top. The AMS for unconsolidated sediments developed by P. Wassmer et al. (2010) offers a new dataset for tsunami deposits. Also, we are presently extending AMS applications to riverine sand deposits, and to volcanic geomorphology, in order to notably improve the knowledge of deposition conditions of lahars and block-and-ash flows.

This research has been partially funded by the French Government in the aftermath of the boxing day tsunami in 2004, by the DIPT. The program was lead by Franck Lavigne and co-directed by Raphael Paris. Preliminary interpretation of the AMS were also fostered by J.-L. Schneider and laboratory analysis would not have been possible without the help of Olivier Averbusch. The authors are also in debt to Hervé Regnauld, Franck Lavigne and an anonymous reviewer who helped improving this manuscript.

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